Single faced corrugated paperboard webs are made on a single facer machine. A liner is attached to the crests of the corrugations to form double faced corrugated paperboard at a glue machine. At the glue machine, the web passes between an adhesive applicator roll and a rider roll. The amount by which the rider roll compresses the corrugations establishes the amount of adhesive applied. Accordingly, the rider roll must be ballasted to apply either a constant predetermined pressure or to maintain a constant predetermined gap between itself and the adhesive applicator roll.
The effective pressure can be maintained either by a pressure control arrangement as described in U.S. Pat. No. 2,876,734 or by monitoring the gap between the rider roll and the adhesive applicator roll. To maintain a uniform gap so that slight compression of the single face occurs as it passes between the rolls, the thickness of the web must be sensed. Thus, there is need for sensing apparatus capable of acting on the corrugated side of the web for measuring the thickness of the web as the web is moving at high speed so that appropriate adjustments and/or stopping of the web can be attained. The flexibility, the ease of deformation and the undulated nature of the corrugated side of the web complicate the problems in designing suitable sensing apparatus.